ftp.barnyard.co.uk/2015.02.ftp.barnyard.co.uk.tar/ftp.barnyard.co.uk/cpm/walnut-creek-CDROM/CPM/HEATH/ZB4MHZ.DOC

Text File | 2000-06-30 | 5KB | 107 lines

            ZB4MHZ.DOC

        P. L. Kelley -  [phone]
             05/29/82

    The file ZB4MHZ.ASM modifies Percom's ZBIOS.SYS and
ZFOR.COM for CP/M to operate with a 4 MHz CPU on the H89. You
will need to modify the CPU board to operate at 4MHz.
Instructions on how to do this can be found in the December 81
H-Scoop or obtained from Najay Systems, 3136 Vermillion Street,
West Covina, CA 91792. You should be warned that some
instructions describe modifications that are specific to the pre
FCC board (the one that is used with the H-19). The new board
used with the H-19A is not identical in the area of the board
where the Najay modifications are made. It was found that the
description given of the location of the clock pullup resistor
and the lines differed from the layout on the FCC board.

    Why 4 MHz? Speed of course. In particular, I was very
disturbed by the very, very slow warmboot of a "normal" double
density Percom system (256 byte physical sectoring). I decided to
go 4 MHz hoping I could buy at least a factor of 2 on disk I/O
reasoning that the CPU might be able to keep up with Percom's
non-interleaved sectoring. The non-interleaving of sectors caused
the disk to rotate once per sector accessed; it took about 3
seconds to read a 4K (256 byte physical sectors) track. On the
way to 4 MHz I discovered Percom's undocumented "extended"
density (1K byte physical sectors). For a 32K file using normal
density the 4MHz system takes 18 seconds to load the file while
the 2 MHz system takes about 30 seconds; using extended density
the 4 MHz system takes about 5 seconds while the 2 MHz system
takes 10 seconds. In any case, at 4 MHz and using 1K sector size
the H89 is now acceptably fast based on my earlier experience
with other 5.25 inch CP/M systems.

    What is the Percom 4MHz problem? It was not obvious at
least from the approach I took. After modifying the CPU and
making certain that it was running properly by using a 4MHz
modification of Heath's H-17 CP/M I discovered the system would
cold boot from Percom's zrom but do no disk I/O. Using a Z-80
version of Ward Christensen's Resource program I disassembled
both the zrom and ZBIOS.SYS. Much of the code was similar which
puzzled me. I finally discovered a slight difference in the delay
between the interrupt command and the execution of a new command.
There was a MOV A,B in ZBIOS.SYS instead of a POP PSW which was
in zrom. This difference is 6 clock cycles out of approximately
100 in the zrom version. It took me a long while to believe that
an 6 persent difference was that crucial. If I had taken the
straightforward approach of just doubling the timing loops in
ZBIOS I could have saved myself some time. It was also necessary
to add a slight delay on return from the interrupt routine when
writing single density soft sectored disks.

    The changes to ZFOR, Percom's formatting program, were
relatively simple after understanding the 4 MHz problems with
ZBIOS. The tricky part was the routine to synchronize to the
holes when formatting hard disks. I found the appropriate counter
value was not a simple multiple of Percom's value. Either
Percom's value is wrong or my drives are off speed since I had
trouble formatting hard disks at 2 MHz.

    Are there remaining problems? At this point I cannot
cold boot at 4 MHz hard sector or single density soft sectored
disks. This is not surprising for hard sectored disks since
the H17 rom must be modified for 4 Mhz operation. I do not plan
to try to solve this problem a I am quite happy to live with
booting from double density or extended density disks.

    The information Percom gave me about their system was
incorrect. The minimum configuration needed to run the Percom
board as a stand alone soft sectored controller is:

    1. 32K ram
    2. I/O rom (444-61)
    3. H17 rom (444-19)

The H17 controller board is not needed except to make the initial
CP/M disk. If you own CP/M for the Heath have your dealer make a
soft sectored Percom system for you or borrow a friend's H17
board to make your own. If you upgrade from an H19 order the roms
at the same time you buy your upgrade kit.

    Included in ZB4MHZ.ASM is an implimentation of the
parallel port, something that Percom has not provided in version
1.00 of their BIOS. It is a simple routine for a Centronics 730
which allows paging and eliminates a linefeed after carriage
return assuming you are using the auto linefeed after carriage
return feature of the 730. It is also possible to cut the
connection between U13 pin 12 and U9 pin 10 and reconnect U13 pin
12 to U9 pin 9. This readdresses the status port for the printer
from E5 to E6. Since the DSR bit is the same on the serial port
as the BUSY bit on the parallel port, this allows much of the
software written for a serial port to be compatible with the
parallel port; in particular, ZBIOS.SYS will drive the parallel
port.

    ZB4MHZ.ASM can be converted from the version to modify
ZBIOS.SYS to the version to modify ZFOR.COM simply by reversing
TRUE and FALSE in two equates in the file. There are options
available in ZB4MHZ.ASM for modifying ZBIOS.SYS depending on
memory size and whether the Centronics 730 driver is desired.
ZB4MHZ.ASM is written for use with versions 1.00 of ZBIOS.SYS and
ZFOR.COM, when and if newer versions become available extensive
modifications to ZB4MHZ.ASM will likely need to be made.